Economic cost of BVD (Proceedings)

Suckling calves are commonly in contact with the breeding herd during early gestation, prior to the time the bovine fetus develops a competent immune system. As a result, PI suckling calves are considered to be the primary source of BVDV infection in breeding herds causing pregnancy loss, pre-weaning mortality and the induction of PI calves in the next generation.

Although PI suckling calves are considered to be the primary reservoir for BVDV in a herd, PI adults can also be present in a herd. Adult PI animals are not as common because mortality of PI calves prior to and after weaning has been reported to be very high due to fatal congenital defects and secondary infections that cause enteritis, pneumonia, and arthritis. However, 17% to 50% of PI calves may reach breeding age in some situations. PI breeding females not only are a source of horizontal transfer of BVDV, but will always produce a PI calf themselves. Witttum et al., showed that 7% of PI calves were born to dams that were PI. Meaning that while PI dams can be a direct cause of a small percentage of PI calves, a vast majority (93%) of PI calves are born to non-PI dams due to transient infection of the dam during gestation.

Male PI calves will occasionally be selected for use as breeding bulls. The amount of BVDV excreted in the semen of persistently infected bulls is very high (104-106 TCID50/ml). BVDV-contaminated semen is an efficient horizontal transmitter of disease from bull to seronegative females. If PI bulls are used for natural service, PI calves are not common, but seronegative cows may not be able to conceive. Once immunity has developed, cows can conceive and give birth to normal (non-PI) calves. If PI bulls are used for AI, all or most seronegative females bred with the semen will become infected although most will not produce a PI calf.

Circulating virus may exist in herds following removal of PI calves although the efficiency of virus transmission via transient infections alone is not high. Limited data in dairies suggest BVDV may circulate in an unvaccinated herd for 2-3 years following removal of all PI animals. The length of time for BVDV circulation in vaccinated beef herds without PI animals has not been reported.

Other sources of BVDV to consider

Embryo Transfer

Embryo transfer is a potential route of transmission of BVDV. If the embryo recipient is PI, vertical transmission to the transferred embryo will occur with the creation of a PI fetus. Although there is no evidence to suggest that BVDV is present inside the embryos of viremic females, the virus can be present on the intact zona pellucida of PI and transiently infected females and the virus is present at high levels in the uterine environment of PI donors. Established washing procedures will remove contaminating virus, but if these procedures are not followed, BVDV from the collection fluids or virus present on the zona pellucida can be horizontally transferred to a susceptible recipient cow. Vertical transmission from the recipient cow to the fetus can occur resulting in fetal death or the birth of a PI calf. BVDV infection of the recipient cow and fetus can also occur when both the donor and recipient are free of BVDV if BVDV-contaminated fetal serum is used in the embryo transfer process or if contaminated liquid nitrogen is in direct contact with embryos.

Other ungulate species (domestic and wildlife)

Other ungulate species may be potential sources of BVDV to susceptible cattle herds. Transmission of BVDV between sheep and cattle has been demonstrated, but the importance of this transmission has not been established. BVDV has also been isolated from pigs, but again, the importance of pigs as a source of the virus to susceptible herds is not established. Deer seropositive to BVDV have been identified in North America and Europe. And at least one case of a captive deer in Europe being persistently infected with BVDV has been documented.

Fomites

Fomites may serve in the transmission of BVDV from PI cattle to susceptible animals. A 19-gauge needle was able to infect susceptible cattle with BVDV when used IV within 3 minutes of drawing blood from a PI animal. Nose tongs were able to infect susceptible cattle with BVDV when used for 90 seconds within 3 minutes of being used in a PI animal.

Economic cost of the presence of PI cattle in a herd

The presence of PI animals in a breeding herd can result in decreased pregnancy percentage compared to herds with no PI calves. This decreased pregnancy percentage could be due to ovarian dysfunction, failure of fertilization, early embryonic death, and/or mid-gestation fetal loss in cattle acutely infected with BVDV. Grooms et al found that BVDV could be isolated on days six and eight following infection with noncytopathic BVDV in ovarian stromal and macrophage-like cells, and oophoritis was evident from 6 to 60 days post infection. McGowan et al reported that conception percentage, determined 20 days after insemination, was lower in heifers intranasally-infected with BVDV nine days before insemination compared to controls (44% vs 70%; P=0.055). In addition, conception percentage was numerically lower in heifers exposed to a PI cow-calf pair four days after insemination than in unexposed controls (60% vs 79%; P=0.255). The intranasally-exposed heifers also experienced significant embryo-fetal loss, resulting in a pregnancy percentage, determined 77 days after insemination, significantly lower than controls (33% vs. 79%; P=0.018). Rufenacht et al found that dairy cows infected with BVD during the first 45 days of gestation (as indicated by seroconvertion to BVD during that time frame) had the same conception percentage as cows that either had previous exposure to BVD (seropositive for BVD by start of the trial) or that were not exposed to BVD during gestation (no seroconversion during trial). Similarly, BVD-exposure status did not influence late gestation pregnancy loss (>210 days). However, cows infected with BVD during mid-gestation (days 46-210) had greater pregnancy loss compared to those that were seropositive prior to breeding or that were not exposed to BVD during mid-gestation (pregnancy loss of 15.8% vs 6.1%; OR=3.1; P<0.02).

In addition to decreased pregnancy percentage, reproductive efficiency can be decreased due to fatal congenital defects following fetal infection of BVDV between 100 and 150 days of gestation. The teratogenic lesions associated with fetal infection with BVDV include microencephaly, cerebellar hypoplasia, hydranencephaly, hydrocephalus, defective myelination of the spinal cord, cataracts, retinal degeneration, optic neuritis, microphthalmia, thymic aplasia, hypotrichosis, alopecia, brachygnathism, growth retardation and pulmonary hypoplasia.

Studies have reported a pre-weaning mortality proportion for PI calves of 20% to 83%, which can result in substantial loss in herds with a high prevalence of PI calves (i.e. 10%). However, overall mortality percentages between herds with or without PI calves were not statistically significantly different because of the low prevalence of PI calves within most PI-positive herds. In herds with PI animals present, Larson et al., modeled scenarios both with and without a 10% negative effect on pre-weaning mortality percentage; and found that the greatest economic loss due to the presence of PI animals was decreased calving proportion with a lesser loss due to pre-weaning mortality.

The effect of introducing a PI animal into a beef herd (confined breeding and calving seasons) depends on the timing of the introduction relative to the breeding season and the resulting immunologic status of the herd during early gestation. Even in the absence of vaccination, the number of PI animals and the amount of BVDV infection in a herd seems to be self-limiting. A likely scenario for a BVDV-exposed herd is to experience an initial peak of disease and then in subsequent months and years, to experience low-level chronic reproductive losses. If a PI animal enters the herd either by birth or by purchase near the start of the breeding season, a high percentage of the herd may not be immunologically protected to the degree necessary to prevent viremia, conception failure, abortion, or fetal infection. Once the PI animal is in contact with the breeding herd for a long enough period of time, the majority of the herd should become infected and seroconvert. Seropositive animals are less likely to have conception failures, abortions, or infected fetuses compared to seronegative animals. If no intervention is applied to the herd, the number of susceptible females the following year should be greatly decreased and the number of abortions and infected fetuses (both persistently infected and immunocompetent) should decrease. A model developed by Cherry et al. indicates that in continuous calving dairy situations, the proportion of PI animals in the herd will reach equilibrium of about 0.9 to 1.2% in herds with no BVDV control procedures.

Evaluating BVDV intervention with a risk management perspective – appropriate for BVDV in a cow-calf herd

When veterinarians are confronted with designing prevention or control strategies for diseases that can cause extensive biologic and economic losses, but that occur sporadically or rarely (such as BVDV-associated disease in cow-calf herds), a risk management perspective may be appropriate. Bovine viral diarrhea-associated disease prevention in the form of facility (i.e. fence) construction, annual or more frequent vaccination, and/or test-and-cull has an ongoing cost regardless of whether or not disease risk is realized. For diseases with sporadic or rare occurrence and subsequent biologic and economic cost, this ongoing expense of prevention may equal or exceed the long term cost of the BVDV-associated disease. Therefore, if using a straight-forward partial budget to evaluate the cost-effectiveness of the control strategy, the most appropriate answer may appear to be to avoid the health intervention cost and accept the disease cost. However, by taking a risk management approach, other factors such as the economic position of the farm to absorb an occasional severe disruption in cash flow are considered. If an individual farm could not survive the costs of an uncommon but realistic disease event due to BVDV (or has high risk aversion); that entity may elect to incur the annual expenses for disease prevention as a risk management decision rather than to optimize long-term resource allocation. In other words, some entities may elect to incur greater disease control expense than expected disease loss over the long-term to avoid or reduce the risk of a short-term financial loss that would result in business failure. Confronted with the same situation, a farm or entity with substantial financial reserves (or high risk acceptance) may elect to risk short-term financial losses in order to optimize long-term resource allocation.

One way to evaluate potential health interventions based on a risk assessment perspective is to compare the probability of experiencing a financial loss equal to or exceeding some predetermined reduction in net return between different disease prevention strategies. Then based on the annual costs of each strategy, the decision maker can make an informed selection based on their level of risk avoidance.

Smith and Sanderson (2007) designed a stochastic modified Reed-Frost disease model of introduction, spread, and impact of BVDV in a cow calf herd. The model was designed to determine the efficacy and economic value of vaccination, testing and removal of PI calves prior to the onset of the breeding season, and testing of herd additions as interventions for the prevention and control of BVD in cow-calf herds. The model demonstrates how pathogen, environmental, and animal factors can each influence the extent of infection and disease resulting from the introduction of an infected individual into the herd. The model design accounts for factors affecting the risk of disease including immunity from vaccination or natural exposure, the presence of PI cattle, the R0 value of BVDV, vaccine efficacy, and vertical transmission risk to predict the impact of introducing a persistently infected individual into the herd.

Summary

Bovine viral diarrhea can cause significant economic loss to beef cow herds by negatively affecting reproduction and calf health. Persistently infected animals are the primary reservoir for BVD virus and preventing the creation of PI calves is the focus of BVD control programs.

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